This invention relates to polishing of substrates, and more particularly to retaining ring apparatus for retaining a substrate during polishing. More specifically, this invention relates to an improved retaining ring which allows for the secure attachment to a carrier head without introducing undesirable materials into the polishing environment and with minimal or no deformation at or near the fastener locations.
Chemical mechanical polishers are used in several applications including the manufacture of integrated circuits where they provide the silicon wafer substrates with a smooth flat finish during the deposition of conductive, semi-conductive and/or insulative layers. The semiconductor wafer is placed on a carrier head which holds the wafer using a combination of vacuum suction or other means to contact the rear side of the wafer.
A retaining ring around the edge of the wafer retains the wafer on the carrier head. The front side of the wafer is then contacted by a rotating polishing pad that polishes the outermost surface of the wafer to a flat smooth surface. During the polishing, the carrier head and retaining ring assembly press against the substrate and the rotating polishing pad. The movement of the polishing pad across the surface of the substrate causes material to be mechanically and chemically removed from the face of the substrate.
In the polishing of semiconductor wafers, it is important that the equipment and materials used in the process, including the retaining ring and the materials used in the retaining ring, are compatible with each other and with the chemical and material constraints inherent to the semiconductor device. Those skilled in the art recognize that a silicon wafer with partially constructed devices, such as memory chips or microprocessors, are inherently vulnerable to negative chemical processes such as corrosion, electrostatic emission, physical damage by contact with foreign objects, contamination with foreign materials from equipment component wear and degradation, by-products from chemicals and materials used in processes, and other dilatory factors and processes inherent in chemical mechanical processing.
When polishing conductive materials such as tungsten, copper, conductive polymers, and the like, the process environment must be controlled to minimize the propensity of high-purity metals to degrade when exposed to surface contamination. One method of minimizing such contamination is the use of materials that are not chemically reactive in the construction of the polishing equipment. Because the polishing of conductive materials generally involves using chemicals that react with metal surfaces, it is desirable to minimize or eliminate exposure of any metallic components in the chemical mechanical polishing environment. Historically, this has partially accomplished by constructing components of the equipment from specially designated plastics that are non-reactive but provide near-metallic strength. This method has been successful where, for example, the physical properties of the plastics, such as the heat stability, durability, ability to withstand friction, etc., were suitable substitutes for metal in the polishing process and equipment. Where the substitution of plastic for metal has not sufficed, it has been necessary to design processes that allow for some inherent contamination during processing.
While the problems inherent in polishing conductive materials seem apparent, there are also significant difficulties in polishing non-conductive materials such as doped oxide materials, including tetraethyl orthosilicate (TEOS), borophosphosilicate glass (BPSG), and other layers deposited using chemical vapor deposition, electrodeposition, epitaxy and other deposition methods. As a result, the process environment must also be controlled during the polishing of these materials.
While non-conductive materials tend to be more stable than conductive materials, they are nonetheless subject to damage during processing, including surface damage, contamination by contact with foreign matter, chemical contamination and ionic contamination. In the case of ionic contamination, for example, the non-conductive layers, particularly those involving device isolation processes such as those occurring early in the semiconductor device creation process, must not be exposed to ionizing materials such as sodium, potassium, and the like These ions, sometimes called mobile ions, are extremely detrimental to semiconductor devices. To limit the exposure of the wafer surface to mobile ions, the process space is, where possible, constructed of materials that do not react to the chemicals used during processing. For example, when polishing non-conductive material, basic or high pH chemicals are typically used. Ideally, the chemical mechanical processing area would not have any exposed metallic equipment due to the inherently reactive nature of metallic materials to non-conductive polishing chemicals.
It would be highly desirable to provide a one-piece retaining ring assembly for use in chemical mechanical polishing which did not introduce undesirable materials into the polishing environment so as to limit the exposure of the wafer surface to mobile ions, while at the same time being sufficiently rigid to be used as a substitute for existing chemical mechanical polishing applications. The result would be a significant improvement in the overall polishing process. The ring assembly of the present invention obtains these results.